The present invention relates to water heaters in general, and more particularly to water heaters with multi-flues, at least one of which is condensing.
Heat exchange between a liquid and a gas is a process which has many industrial and domestic applications. Perhaps one of the most widely used applications of heat exchange between a gas and a liquid is in heating water. Typically a water heater has a tank which holds the water to be heated, and a burner producing combustion gases. The water is heated by the combustion of fuel with air in the burner to produce the combustion gases which heat the water in the tank by passing through one or more flues or tubes which extend through the water tank. Two considerations which are paramount in the design of a water heater are durability and efficiency. To conserve resources and limit the production of carbon dioxide, efficiency has become paramount in all devices using combustion to generate heat. Approximately 7-10% of the heat available in burning a hydrocarbon fuel such as natural gas, or No. 2 fuel oil, is contained in the latent heat of the water produced during combustion. Practically, only 85-89% of the heat of combustion can be transferred without condensing the water produced by combustion. The water as vapor contains the energy necessary for the phase change between liquid and vapor, which for water, is about 1000 Btu/lb. The latent heat of the water vapor in the combustion gas, must be condensed to extract the latent heat. This increases the complexity of the design due to the corrosive nature of liquid water as opposed to steam. Furthermore, it is necessary to control the combustion level to prevent condensation of water before it has reached the flue structures which are designed to condense the steam, and to properly drain the water when it condenses so as to not block the free flow of the combustion gases.